This is a proposal to study the electrical stimulation of cat auditory nerve fibers, in order to improve our understanding of the neural response driven by cochlear implants. Psychophysical studies in cats have demonstrated their ability to perceive electrical stimulation of the cochlea at levels lO to 50 dB below those typically used in single-fiber studies; the goal of this research proposal is to take neurophysiological data that characterizes the neural response at these lower levels. This single-fiber data acquired from this study will allow the construction of a linear- systems model of the neural response to arbitrary, small amplitude waveforms (Aim l) and will also quantify the significance of changes in the neural response driven by changes in the excitation waveform shape (Aim 2). Sinusoidal stimuli will be used initially (Aim l), because of their fundamental importance in the development of a linear model, but these results will be generalized and compared to the response driven by square wave stimuli in Aim 2. Future studies will be then be able to address modifications of these elementary waveforms that are typically used within cochlear implant stimulation paradigms based on either compressed analog or interpulse interval codes. The specific aims are as follows: Aim l: Characterize single-fiber input-output functions at firing probabilities below 0.5, in response to sinusoidal stimuli of various frequencies. Extracellular recordings of the spike activity from single cat auditory-nerve fibers will be used to characterize input-output functions and the dynamics of the response driven by electrical stimulation at near threshold levels. All stimuli will be delivered by a single electrode placed within the scala tympani of acutely deafened cats; this animal model minimizes the effects of cochlear location and pathology. Stimulation frequencies of 1 kHz and below will be used to evoke a spike with probabilities in the range of 0.5 to 0.001. In the past, similar experimental studies have focused on measurement of the stimulation intensity required to excite a neuron with probabilities near 0.5. Aim 2: Quantify changes in the neural response driven by sinusoidal stimuli vs. pulse-train stimuli of the same frequency. A single fiber's response will be compared across stimulation waveform type. Both sinusoidal and pulse-train waveforms will be presented at the same level and frequency; the neural response will be can then be analyzed for changes in the response that track the change in stimulus. Measurement of a single neuron's response to two similar waveforms will drive the development of a model which predicts the discriminability of two stimuli, which is independent of the linear system's approach to be developed from the data taken previously. They will also allow a analysis to be performed and to quantitatively access the significance of changes in the neural response.